Twelve Port Heads
The introduction of the blue engines brought a completely new head design, and it some ways it was an improvement over the old red head, though it still could hardly be considered as high-performance. But where the red head sucked, the blue head sucked slightly less.. Obviously, there is now an individual port for each cylinder, and the exhaust dividers now extend to the port face so extractors will work properly. The valves are bigger, and there is improved cooling with some additional water holes. When fitting a 12 port head to a red block use a gasket as a template to drill matching holes in the block deck. The new intake ports are higher and narrower so the air flow now has a less severe angle to negotiate, though they are still too low and too small. The intakes will probably look very familiar; they resemble a slightly smaller small block Chevy port.
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| Twelve port head showing taller individual intake ports. |
Common wisdom has it that a stock 12 port flows about the same as a Bathurst style or YellaTerra 9 port head, though the few I've tested have flowed a bit less. They do have the potential for a fatter torque curve than that of the 9 port head though. The reason for this is that the individual ports allow the use of a true individual runner intake manifold such as is used with the EFI setup or six throat IR carb setup. The peak horsepower thus obtained probably won't be much different to the 9 port but the extra midrange will certainly boost performance. There is potential for flow improvement with these heads as well though they don't quite have as much scope for improvement as the 9 ports - although thats partly because the stock 9 port heads are so extraordinarily bad.
Basically there are two different chamber configurations: the small chamber as used on the 173 and the large chambered 202 head. There are also two different intake ports: the VK EFI head had a laid-back short turn and different floor profile that improved the flow significantly over the 173 and carbureted 202 heads.
The blue/black intake ports are quite well balanced compared to the nine-port. By "well-balanced" I mean that unlike the nine-port, where you can pick up a lot of flow simply by cleaning up the bolt boss and short turn, there is no one single part of the twelve-port intake that stands out as a primary restriction. To get large flow improvements from these heads requires metal removal from almost the entire port. The upside of this is that even after all the port work is done the port efficiency (and therefore flow velocity) will be relatively high. If you want to make the most of them it's probably best to leave the port work to an experienced specialist.
If you do decide to port one of these heads yourself, you must have access to a flow bench. These ports are sensitive to subtle changes (at least compared to a 9 port) and it would be very easy to do a lot of work for little gain without flow verification. The flip side to this is that very good improvements are possible without making the port excessively large. Start by installing new, oversize intake valves of about 1.75", using the usual 30/45/60deg cuts. Like the 9 port heads, these intakes seem to like a fairly straight sided throat that is taken almost right out to the inner edge of the seat. This will also help increase the bowl volume, as will raising the roof, tapering it back from the throat just past the valve guide. Don't enlarge the port at the gasket face at all; don't even square it up. There's a fairly big change in CSA from the gasket face into the port proper and any grinding here will just make it worse. Once you get about 20-25mm into the port though, you can remove a fair bit of material from both sides and the roof to pick up some area. The outer wall can be made quite straight (or even a little bit concave to partly match the curve in the opposite wall) while the common wall can also be moved over quite a bit. Also try to widen the port as much as the wall thickness allows around the valve guide area. The standard port has a minimum CSA that is very very small, and opening this area up is essential to making use of the flow potential of the oversized valves.
These heads respond very well to laying back and widening the short turn radius, and if you get the chance to look at an EFI head you will see how the factory modified the short side and the floor to pick up quite a bit of high-lift flow. Be very careful though with the non-EFI heads; if you push it back any less than 48 - 50mm from the gasket face you will break through to water. Make templates so you can lay back the SSR as far back as you safely can while making the actual radius as large and gentle as possible. Flow also improves with widening the short turn but be careful again not to cut too much from the corners as the wall will be thin here.
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| Sectioned blue 12 port intake. Port had been roughed out and flowed over 160cfm but broke through at short side - oops! Note the thickness around spring seat, bowl and chamber where unshrouding has been done. |
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| This shot shows the wall thickness remaining in the common wall after fairly extensive porting. There is enough meat here to increase the CSA substantially. |
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| Non EFI heads respond very well to laying back the short turn, however you will strike water at about 50mm from the gasket face. |
The exhaust port is also improved over the nine port, and as with the 9 port heads exits the head pointing slightly downwards. This might sound a bit odd, but I guess the idea behind it is to ease the transition into the exhaust manifold. The carb versions have lumps in the roof of the exhaust ports where the air injection nozzles screw in. These can be plugged and ground back. Be careful not to enlarge the exhaust port as it will already be about the right size for a 1.5" primary tube, however it won't hurt to slightly streamline the somewhat chunky valve guide boss and open up the throat and bowl areas.
These heads have a reputation for being a bit prone to cracking, and the quality of the castings certainly doesn't look that flash. I think that the cracks might have more to do with the cars they were installed in than a problem with the head though. The old Commodores ended up having the radiator top tank lower than the cylinder head, so it was easy to get a pocket of air or steam trapped in the head, especially if the cooling system wasn't bled properly. The Nissan engines also suffered cracked heads in the same type of car, but were trouble free in other cars. The bottom line is this: get the head crack tested before you invest time or money in them, and if you have one of the old Commodores bleed the cooling system thoroughly.
The combustion chambers are very similar to the 9 port heads, and have the same problems with valve shrouding and the same step at the top of the cylinder bore. If you plan on using factory 12 port heads I suggest taking a look at the VK EFI version; as mentioned earlier they have better flow characteristics than the other heads (by virtue of a multi-angle valve seat and face, and a reshaped short-turn and floor) plus they don't have the air injection humps intruding into the exhaust ports. The downside is the big open chambers that do little for compression or squish. Unless you are building a very mildly cammed engine it's likely that the 202 chamber will be a bit big. The blue 173s have a much better chamber, and these heads can be ported to provide much the same flow as a ported EFI head to give the best of both worlds - BUT be very wary of that water passage in the short side turn.
YellaTerra can still provide 12 port heads, and if you can afford them
they are the pick of the 12 port Holden-style heads.
They look quite good, with oversized valves and improved porting with
thicker walls. At present I think only the aluminium versions are still
available. At least 200cfm should be possible with these heads (after
porting), a figure that is very very difficult to get achieve with the
Holden heads. Efficiency should be good, as should be in-car performance.
Keep in mind
that headwork doesn't come cheaply, so it's quite possible that a
complete new head will be cheaper than rebuilding and modifying a
used one. The biggest single disadvantage to using the 12 port
would have to be the scarcity of good intake manifolding. The
factory EFI works quite well within its limits, and even the factory 2 barrel
manifold would be acceptable for a mild daily driver, but what
these heads really like is a good, true IR (individual runner) type
manifold with individual carb throats. A good IR setup on these motors
will be very quick, and should make good power over a wide rev range. Select your manifolds
carefully; some are very badly designed will give disapointing
results. In particular approach anything that is claimed to fit
both 9 and 12 port heads with suspicion. To make the most of a 12 port
head it's quite likely that you'll have to build your own manifold. Get
it right though and you'll probably enjoy perormance that belies the modest
flow numbers.